Dialysis: Gradients, Counter-Current Flow and Potassium
A patient with kidney failure attends a dialysis unit three times a week. During a session, blood is passed through a dialyser containing many fine tubes made of a partially permeable membrane. Dialysate fluid flows on the outside of the tubes in the opposite direction to the blood. The dialysate contains glucose, sodium, potassium and chloride at concentrations similar to normal blood plasma, but no urea. A nurse records the concentrations in the blood entering and leaving the dialyser: urea 28 mmol/dm³ entering and 9 mmol/dm³ leaving; glucose 5.0 entering and 4.8 leaving; potassium 6.2 entering and 4.5 leaving. The patient's urine output is 200 cm³ per day. The nurse explains that the patient must limit potassium-rich foods.
1. Explain why urea leaves the blood but most glucose remains in the blood during dialysis. Use concentration gradients in your answer.
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Urea is at 28 mmol/dm³ in blood entering and zero in the dialysate, so there is a steep concentration gradient. Urea is small enough to pass through the partially permeable membrane and diffuses out until the gradient is reduced. Glucose is about 5.0 mmol/dm³ in blood and the dialysate is made with a similar glucose concentration, so there is little net gradient. Glucose can cross the membrane, but with little difference in concentration only a small amount is lost; the fall from 5.0 to 4.8 is consistent with a small gradient. Making the dialysate glucose-free would create a large gradient and waste glucose.
2. Explain why the dialysate flows in the opposite direction to the blood.
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Opposite flow is counter-current flow. It keeps a concentration gradient along the whole length of the dialyser: blood with the lowest urea meets fresh dialysate with no urea, and blood with the highest urea meets dialysate that has already picked up some urea. This maintains diffusion of urea out of the blood for longer. If both flowed in the same direction, the concentrations would equalise near the start and less urea would be removed. The same arrangement helps keep glucose loss small.
3. The patient's urine output is only 200 cm³ per day. Explain why the nurse restricts potassium-rich foods and why the dialysate is not potassium-free.
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The failing kidneys cannot excrete potassium efficiently, so potassium can rise in the blood. High blood potassium can disturb the heart rhythm, so intake must be limited. The dialysate contains a normal plasma-level of potassium rather than none because a potassium-free fluid would create a very large gradient and remove too much potassium. That could cause low blood potassium, which also affects muscles and the heart. A moderate gradient removes the excess while avoiding a dangerous swing.